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The person I bought my Trailblazer from removed the check engine light without telling me but I had a feeling it was on after a while so I went to AutoZone to check it and it threw code p0172 P0171 P0340. I have real bad vibrations and bad idle and really bad fuel economy. I'm thinking of replacing both O2 sensors and the camshaft position sensor. Is there anything I should know before doing this? I'm getting the cheaper parts on rock auto. Or could it be something else?
 

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How wonderful of that person! May he be infested with the fleas of 1000 camels!

OK, so you have a P0171, P0172, and a P0340.

In all honesty, the P0171 and P0172 are opposite of one another relative to the problem. One indicates that the engine is running too lean (P0171), and the other indicates that the engine is running too rich (P0172). It's kind of hard for the engine to run both ways at the same time.

Now the P0340 code is telling you that there is a malfunction with the camshaft position sensor's circuit. It does not say that there is a problem with the camshaft position sensor.

Until you (we) do some more diagnostic testing, do not buy any replacement parts. Around here we prefer to Diagnose, Diagnose, Diagnose, and then buy the correct part if need be.

First, the P0340 code. Check to make sure that the wires to the camshaft position sensor electrical connector are securely set in the connector and that the insulation is intact. Next, remove the electrical connector and make sure that the internal contacts in the connector and the sensor are both clean. Now, do those 2 steps again only this time do it for the CCT solenoid.

Also, here is a link to a great webpage on how to test the camshaft position sensor:


You will need a decent digital multimeter (DMM) to do the testing. Think of something in the $30 - $40 dollar range from Home Depot or Lowes.

Also, you are going to need a decent OBDII code reader/live data scanner in order to work on this or any other modern vehicle. It is a necessity. If you don't have one and would like a recommendation, we can handle that and it is relatively inexpensive for what you get.

Now to the tough one. Is the engine running rich or is it running lean, or could it be running both? First some questions.

Did the current driveability difficulties come on all at once, or did it happen some time ago and the symptoms just kept getting worse so you stopped by Autozone to get the codes read?

Have you replaced any other parts or filters since this began, or have there been any other repairs?

Do you have a DMM and a decent OBDII code reader/live data scanner? (I'm going to guess no since you stopped at Autozone and asked them to read the codes for you.) Here is a link to a Bluetooth OBDII dongle and if you have an android device, you can download the Torque Pro App from the Google Play Store.


Now, please check the wires going to both the O2 sensors. One will be sticking out from the exhaust manifold in the engine compartment and the other will be down below the car just a little past the catalytic converter. You want to be looking for wires that have burned or melted insulation, wires that are rubbing against anything metal, or broken wires.

OK, next I need to get a fuel injection pressure gauge. Go to Autozone, O'Reilly or Advance and they all have a loaner tool program for "specialty tools". Leave a deposit, take the tool and do test, return the tool, get you deposit back. The tools are actually pretty nice tools for loaners.

You'll need to measure the fuel pressure of the fuel pump. The test port is underneath the vehicle right next to the fuel filter. This is located on the driver's along the frame rail in front of the fuel tank. While you are down there go ahead and change the fuel filter if you have not done so since you have owned this TrailBlazer.

The fuel pressure needs to be in the range of 50 - 57 psi. If it is below 50 psi, your fuel pump is on it's way to the grave yard and needs to be replaced soon! NOTE: there are a lot of cheap and inexpensive fuel pumps out there that are junk!!!! Stay away from them. I used to recommend ACDelco or Delphi fuel pumps, but now I only recommend ACDelco fuel pumps. It seems Delphi pumps have gotten cheaper in quality. I'm not sold on Bosh fuel pumps despite Bosch being an OEM supplier to most of the auto manufacturers.

Now you need to check the fuel pressure regulator (FPR) for proper operation (or is the internal diaphragm ruptured). The easiest way to do this is to get a 6 foot section of 3/16" id vacuum line, remove the vacuum line from the fuel pressure regulator, put one end of the 6 foot section of vacuum line on it and then stick the other end of the vacuum line in your mouth. Now, carefully suck to create a vacuum. It won't take much and remember, if the internal diaphragm is ruptured, you could wind up with a mouthful of gasoline. So, easy does it. You'll now within a second or two if the diaphragm is ruptured. If you are lucky and the diaphragm is ruptured, gas should run out of the FPR's nipple.

My thinking is that if the FPR is defective, it will cause a lot of extra gasoline to go through the engine and cause the P0172 code since the engine will be running rich.

Now the other thing you need to do is check over the engine for any vacuum leaks that could cause the lean condition which sets the P0171 code. A couple of ways to do this. One is spray some carb cleaner around the intake manifold, the throttle body and wherever you see a vacuum line. If there is a vacuum leak, you'll hear the engine speed increase as the vacuum leak sucks in the extra "fuel". The other way to do this is to use an OBDII code reader/live data scanner and watch the Short Term Fuel Trim values while spraying the carb cleaner around. If you spray the carb cleaner where a vacuum leak is, you will see the Short Term Fuel Trim number drastically change in short order (for example it will go from -3.00% to -15.00% in 2 seconds, and then go back to -3%).

Also, do you know when the spark plugs were last changed? How many miles are on your TrailBlazer?

Now regarding parts. Purchasing them from Rockauto is fine. I buy most of my parts from them. Just stay away from the parts that are listed in the "Economy" section. Those are more than likely white box, generic parts that are manufactured with 2nd or 3rd rate materials with little or no quality control. Stick with parts from ACDelco, GM Genuine, Delphi (except fuel pumps), Standard Motor Products (non T series), NAPA Echlin, BWD (non P series), Denso, Fel-Pro, Raybestos, Wagner, Dana-Spicer, Moog, Gates, Remy, TYC, Continental, etc. I also tend to shy away from parts from Airtex, Wells Vehicle Electronics (WVE) and other companies I've never heard of. Lately, I've also shied away from Carquest engine management parts from Advance Auto Parts because they dropped their long time supplier (which was Standard Motor Products) which supplied top tier parts, and are now getting parts from the lowest bidder, so who knows what their quality is. I've also shied away from Autozone's Duralast engine management parts because they used to be manufactured by WVE, and lately it appears they have a new supplier and I do not know who that is or their overall quality.

Regarding spark plugs, these engines are damn finicky when it comes to spark plugs. Use only ACDelco 41-103 iridium plugs.

Now a bit about counterfeit parts. Don't buy parts from ebay unless you know exactly what you are buying. A significant (and very high) percentage of parts sold on ebay are counterfeit. If the price is too good to be true, it is likely counterfeit. While Amazon is a little better than ebay, I've been reading posts on other auto forums from a rather large number of people who are being burned with counterfeit parts on Amazon even if it says Sold and Shipped by Amazon. Apparently there is a lot of commingling of parts from various sellers that go into the same parts bin (so real and counterfeit are apparently being mixed).

Good luck and I am looking forward to hearing back from you!
 
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Discussion Starter · #3 ·
How wonderful of that person! May he be infested with the fleas of 1000 camels!

OK, so you have a P0171, P0172, and a P0340.

In all honesty, the P0171 and P0172 are opposite of one another relative to the problem. One indicates that the engine is running too lean (P0171), and the other indicates that the engine is running too rich (P0172). It's kind of hard for the engine to run both ways at the same time.

Now the P0340 code is telling you that there is a malfunction with the camshaft position sensor's circuit. It does not say that there is a problem with the camshaft position sensor.

Until you (we) do some more diagnostic testing, do not buy any replacement parts. Around here we prefer to Diagnose, Diagnose, Diagnose, and then buy the correct part if need be.

First, the P0340 code. Check to make sure that the wires to the camshaft position sensor electrical connector are securely set in the connector and that the insulation is intact. Next, remove the electrical connector and make sure that the internal contacts in the connector and the sensor are both clean. Now, do those 2 steps again only this time do it for the CCT solenoid.

Also, here is a link to a great webpage on how to test the camshaft position sensor:


You will need a decent digital multimeter (DMM) to do the testing. Think of something in the $30 - $40 dollar range from Home Depot or Lowes.

Also, you are going to need a decent OBDII code reader/live data scanner in order to work on this or any other modern vehicle. It is a necessity. If you don't have one and would like a recommendation, we can handle that and it is relatively inexpensive for what you get.

Now to the tough one. Is the engine running rich or is it running lean, or could it be running both? First some questions.

Did the current driveability difficulties come on all at once, or did it happen some time ago and the symptoms just kept getting worse so you stopped by Autozone to get the codes read?

Have you replaced any other parts or filters since this began, or have there been any other repairs?

Do you have a DMM and a decent OBDII code reader/live data scanner? (I'm going to guess no since you stopped at Autozone and asked them to read the codes for you.) Here is a link to a Bluetooth OBDII dongle and if you have an android device, you can download the Torque Pro App from the Google Play Store.


Now, please check the wires going to both the O2 sensors. One will be sticking out from the exhaust manifold in the engine compartment and the other will be down below the car just a little past the catalytic converter. You want to be looking for wires that have burned or melted insulation, wires that are rubbing against anything metal, or broken wires.

OK, next I need to get a fuel injection pressure gauge. Go to Autozone, O'Reilly or Advance and they all have a loaner tool program for "specialty tools". Leave a deposit, take the tool and do test, return the tool, get you deposit back. The tools are actually pretty nice tools for loaners.

You'll need to measure the fuel pressure of the fuel pump. The test port is underneath the vehicle right next to the fuel filter. This is located on the driver's along the frame rail in front of the fuel tank. While you are down there go ahead and change the fuel filter if you have not done so since you have owned this TrailBlazer.

The fuel pressure needs to be in the range of 50 - 57 psi. If it is below 50 psi, your fuel pump is on it's way to the grave yard and needs to be replaced soon! NOTE: there are a lot of cheap and inexpensive fuel pumps out there that are junk!!!! Stay away from them. I used to recommend ACDelco or Delphi fuel pumps, but now I only recommend ACDelco fuel pumps. It seems Delphi pumps have gotten cheaper in quality. I'm not sold on Bosh fuel pumps despite Bosch being an OEM supplier to most of the auto manufacturers.

Now you need to check the fuel pressure regulator (FPR) for proper operation (or is the internal diaphragm ruptured). The easiest way to do this is to get a 6 foot section of 3/16" id vacuum line, remove the vacuum line from the fuel pressure regulator, put one end of the 6 foot section of vacuum line on it and then stick the other end of the vacuum line in your mouth. Now, carefully suck to create a vacuum. It won't take much and remember, if the internal diaphragm is ruptured, you could wind up with a mouthful of gasoline. So, easy does it. You'll now within a second or two if the diaphragm is ruptured. If you are lucky and the diaphragm is ruptured, gas should run out of the FPR's nipple.

My thinking is that if the FPR is defective, it will cause a lot of extra gasoline to go through the engine and cause the P0172 code since the engine will be running rich.

Now the other thing you need to do is check over the engine for any vacuum leaks that could cause the lean condition which sets the P0171 code. A couple of ways to do this. One is spray some carb cleaner around the intake manifold, the throttle body and wherever you see a vacuum line. If there is a vacuum leak, you'll hear the engine speed increase as the vacuum leak sucks in the extra "fuel". The other way to do this is to use an OBDII code reader/live data scanner and watch the Short Term Fuel Trim values while spraying the carb cleaner around. If you spray the carb cleaner where a vacuum leak is, you will see the Short Term Fuel Trim number drastically change in short order (for example it will go from -3.00% to -15.00% in 2 seconds, and then go back to -3%).

Also, do you know when the spark plugs were last changed? How many miles are on your TrailBlazer?

Now regarding parts. Purchasing them from Rockauto is fine. I buy most of my parts from them. Just stay away from the parts that are listed in the "Economy" section. Those are more than likely white box, generic parts that are manufactured with 2nd or 3rd rate materials with little or no quality control. Stick with parts from ACDelco, GM Genuine, Delphi (except fuel pumps), Standard Motor Products (non T series), NAPA Echlin, BWD (non P series), Denso, Fel-Pro, Raybestos, Wagner, Dana-Spicer, Moog, Gates, Remy, TYC, Continental, etc. I also tend to shy away from parts from Airtex, Wells Vehicle Electronics (WVE) and other companies I've never heard of. Lately, I've also shied away from Carquest engine management parts from Advance Auto Parts because they dropped their long time supplier (which was Standard Motor Products) which supplied top tier parts, and are now getting parts from the lowest bidder, so who knows what their quality is. I've also shied away from Autozone's Duralast engine management parts because they used to be manufactured by WVE, and lately it appears they have a new supplier and I do not know who that is or their overall quality.

Regarding spark plugs, these engines are damn finicky when it comes to spark plugs. Use only ACDelco 41-103 iridium plugs.

Now a bit about counterfeit parts. Don't buy parts from ebay unless you know exactly what you are buying. A significant (and very high) percentage of parts sold on ebay are counterfeit. If the price is too good to be true, it is likely counterfeit. While Amazon is a little better than ebay, I've been reading posts on other auto forums from a rather large number of people who are being burned with counterfeit parts on Amazon even if it says Sold and Shipped by Amazon. Apparently there is a lot of commingling of parts from various sellers that go into the same parts bin (so real and counterfeit are apparently being mixed).

Good luck and I am looking forward to hearing back from you!
Thank you for the very detailed response I highly appreciate it. I will definitely be diagnosing all these issues instead of just throwing random Parts at it. I don't know when the spark plugs were changed last but I assume it's been a while. I got the codes red because I just had a feeling something was wrong. And plus the vibration issues in my vehicle was getting worse so I decided to just get a quick scan. I thought it was the motor mounts but it's looking more so like it's this and maybe also the motor mounts. I'll definitely let the form know what's going on with it after I do more Diagnostics and follow your advice thank you
 

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2004 chevy trailblazer_lt
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You are quite welcome!

Sounds like a good plan. If you find that you are in need of new motor mounts, only use GM OEM motor mounts because all the aftermarket ones are junk and will likely fail in short order.

Good Luck!
 
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